Nanoporous, transparent and conducting films are a novel class of materials that exhibit a unique combination of material properties including transparency, conductivity, well-defined three-dimensional porosity and a large specific surface area. Their built-in continuous network of nanoscale voids provides two distinct advantages over conventional solid thin-film transparent conducting oxides (TCOs). Firstly, their continuously interconnected pore volume can be infiltrated with numerous different reactive species to fabricate devices with a large specific surface area. Secondly, some degree of control can be asserted over the effective index of refraction of mesoporous transparent and conducting films by tuning their degree of porosity.
Such materials have been employed as transparent elements within active optical devices. For example, dye-sensitized solar cells (DSSCs) have been fabricated by infiltrating the voids within titania (TiO2) nanoparticle films with a photosensitive dye and an electrolyte. The nanoscale porosity of the TiO2 nanoparticle film provides the large specific surface area required to adsorb sufficient amounts of light-harvesting molecules within the DSSC.
An advantage of nanoporous transparent and conducting films is that their properties can be tuned by controlling their degree of porosity. For example, silica has an index of refraction of about 1.5 at a wavelength of 600 nm, while a nanoparticle film composed of silica nanoparticles with a porosity of 27% exhibits an effective index of refraction of about 1.3.
Numerous methods of fabricating nanoporous transparent and conducting films have been reported in the literature. For example, such films have been fabricated using the Evaporation-Induced Self-Assembly (EISA) process.[1] More specifically, Fattakhova et al. have used poly(ethylene-co-butylene)-b-poly(ethylene oxides), referred to as “KLE's”, as block co-polymers in the EISA process to prepare crystalline and transparent indium tin oxide (ITO) layers with well-ordered, accessible nanoporosity and good electrical conductivity.[2]
As another example, porous TCO films can be prepared by glancing angle deposition (GLAD) (U.S. Pat. No. 6,206,065). In the GLAD process, a thin film of plural columns extending from a surface of a substrate are initially formed. These columns are referred to as “seed posts” which are exposed to a vapor flux incident at an oblique angle. During the deposition, these seed posts grow in a columnar fashion while shadowing the surrounding regions from the incoming flux of vapor. Furthermore, the growth direction of the columns can be controlled by altering the oblique angle from which the vapor flux arrives. The resulting films are anisotropic, porous, and highly structured.
Nanostructured transparent conducting electrodes have also been made by infiltrating a nanoporous structure with a conducting material by means of electrodeposition (U.S. Pat. No. 7,594,982). In this three-step process a transparent conducting layer is first deposited onto a substrate. Secondly, a nano-architected porous film with interconnected pores is deposited onto the transparent conducting layer. Although this nano-porous film must be made from a transparent material, it does not necessarily need to be made from a conductive material, thus as an example, a porous silica film formed using the EISA technique would suffice. In the third and final step electrodeposition is used to fill the pores in with a conducting material.
In yet another example, Lin et al. [5] prepared porous electrodes by using metallorganic chemical vapour deposition (MOCVD) to deposit indium tin oxide (ITO) into the void space of a porous glass matrix. However, the transmittance of these porous glass-ITO composite films was low on account of the visible light being scattered by its micropores. In particular, the optical transparency of the composites was just 32% when measured with an integrating sphere in order to include diffusely transmitted light.
Another method of fabricating nanoporous transparent conducting electrodes is to simply spin- or dip-coat preformed nanoparticles dispersed in solution onto flat substrates.[3,4] Although the transparency of these films can be made to be quite high (>90% in the visible), they must be annealed at high temperatures (of about 500° C.) in order to lower their resistivity.
Recently, nanoporous films have been employed as elements of photonic crystal devices. The possibility of utilizing photonic crystals to manipulate light was first introduced independently by both Yablonovitch and John.[5,6] In short, photonic crystals are a unique class of optical materials that interact with electromagnetic waves causing them to diffract and form interference patterns within the photonic crystal, giving rise to many interesting optical phenomena. For example, photonic crystals can be engineered to possess a photonic band-gap (PBG) over a specified spectral range within which light cannot propagate; in this case incident light will undergo 100% reflection.
Bragg-reflectors, also recognized as one-dimensional photonic crystals (1 D photonic crystals),[8] have been subjected to scientific investigation since the 19th century when Lord Rayleigh showed that light within a specified spectral range incident onto their surface undergo complete reflection.[5] Today, the optical properties of Bragg-reflectors are well-understood, where theoretical advances in the field have shown that 1D photonic crystals can be used to construct optical diodes, which reflect light incident from one side while transmitting light incident from the other side,[10] and omnidirectional reflectors, which completely reflect incident photons over a specified spectral range regardless of their incident angle or polarization.[11,12] Furthermore, owing to their simplicity, Bragg-reflectors have been used to construct numerous optical devices such as Fabry-Perot interferometers,[13] optical resonators for distributed feedback lasers,[14] and optical cavities for controlling the spontaneous emission rates and spectra from emitting media,[15] which is particularly useful for light emitting diodes.
Bragg-reflectors are most commonly fabricated by alternately depositing films from two materials having a differing index of refraction onto a flat substrate. For example, one bi-layer within a Bragg-reflector can be fabricated by first depositing a film from “material A” and then subsequently depositing a film from “material B”. Using this procedure, any number of bilayers can be successively deposited in order to build up the Bragg-reflector to a desired thickness. Moreover, the thickness of the films within the Bragg-reflector can be easily manipulated in order to set the spectral position of the Bragg-diffraction peak at a desired wavelength. Likewise photonic defect states can be easily incorporated into these Bragg-reflectors by appropriately tuning the thicknesses of selected layers during the fabrication process.
The aforementioned method of fabricating Bragg-reflectors can be modified in order to create two- and three-dimensional photonic crystals. One method of achieving this is to deposit the Bragg reflector onto a two- or three-dimensionally patterned substrate.[23] A second method is to etch either a two- or three dimensional pattern through the Bragg reflector. This may be done using electron beam lithography, ion beam etching or other dry or wet etching techniques. The pattern etched through the Bragg reflector may be an array of holes or lines and may be aligned normal to the substrate surface or at an arbitrary angle through the use of, for example, angle ion beam etching.[24] The periodicity of two- or three-dimensional photonic crystals formed in this manner can be set by tuning the thickness of the layers within the structure. Also, photonic defect states can be purposely introduced into these two- or three-dimensional photonic crystals either by altering the thickness of certain layers within the structure or by altering the pattern etched through the structure at specific points.
Recent advances in the field of 1D photonic crystals have involved fabricating Bragg-reflectors from novel nanomaterials primarily to impart added functionality to the photonic material.[16,17] For example, Bragg-reflectors made from nanoporous titania (TiO2) and silica (SiO2) films prepared via template directed sol-gel methods can be employed as color-tunable sensors because the built-in porosity enables the surrounding environment to infiltrate the 1D photonic crystal thereby altering its optical properties, namely the effective refractive indices, and hence its reflectance characteristics.[18,19]
As a second example, photoconductive Bragg reflectors have been made by alternately spin-coating TiO2 nanoparticle films of two different degrees of porosity and subsequently infiltrating their pores with ruthenium dye.[20] Furthermore, Bragg reflectors made of alternating layers of spin-coated SiO2 and TiO2 nanoparticle films have been fabricated on the rear side of dye-sensitized solar cells (DSSCs) in order to reduce transmission losses by reflecting transmitted light back into the cell.[21,22] In this case the porous structure of the nanoparticle films is occupied by an electrolyte thereby providing the necessary pathway for charge transport through the DSSC.
In another example, a conductive inverse opal photonic crystal has been reported by Bielawny et al. [46], where the inverse opal is formed by infiltrating a poly(methyl methacrylate) opal template with zinc oxide. The conductive inverse opal was integrated into a tandem solar cell as an intermediate reflector for enhancing the efficiency of the solar cell. Unfortunately, such devices, in which 3D photonic crystals are templated and then inverted through numerous process steps, are complex and costly to manufacture.